Electrostatic discharge (ESD) protection is becoming ever more important as integrated circuits become smaller and smaller. There are several ways in which to provide ESD protection, each having their own advantages and disadvantages. One common approach is to use a string of diodes where the vertical PNP devices farm a Darlington amplifier and these series PNP devices are coupled with an SCR to form a diode-string triggered SCR (DTSCR). In fact, DTSCRs are becoming one of the dominant ESD protection devices for high frequency I/Os. However, such devices are known to have higher than ideal leakages due to the previously described darlington amplifier. The Darlington transistor network directs most of the current through the substrate at low currents.
The Darlington amplifier is a compound structure consisting of two or more bipolar transistors connected in such a way that the current is amplified by each of the transistors. By way of example, FIG. 1 shows a vertical PNP device, which can result in a Darlington amplifier when two or more diodes are strung together. In the structure of FIG. 1, the PNP diode consists of a p+ region in an N-Well. The N-Well is formed directly on a P-type starting substrate. The p+ region is connected to an I/O pad and the n+ regions in the N-Well are connected to Vdd. The p+ regions (contacts) in the P-wells are connected to ground (GND), and provide current to the backside of the diode (collector region of the PNP).
In operation, current from the P+ diffusion into the N-well leaks into the P-type wafer, resulting in loss of current. That is, some current goes to the collector, compared to an ideal situation in which all of the current goes from the emitter to the base. As more diodes are strung together, though, the loss of current into the N-well becomes amplified resulting in a very lossy device. In some instances, depending on the number of diodes strung together, such structure is known to lose more current to the collector than which is received by the base.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.